Abscesses are caused by bacteria entering the hoof structures. Common causes usually involve trauma to the foot for example standing on a sharp stone.
Symptoms can be heat in the hoof and lameness.
Usual treatment is to dig a hole after locating the abscess to allow it to drain, and to apply a poultice to draw out any remaining puss and prevent further bacteria entering.
Left untreated or poorly treated it could result in an infected Pedal Bone, this will result in surgery or in some cases could prove fatal.